Migration kinetics of the self-interstitial atom and its clusters in bcc Fe

Self-interstitial atoms (SIAs) and SIA clusters are produced in displacement cascades during irradiation of a material with high-energy particles. The migration kinetics of such defects are a critical factor in controlling microstructure evolution and the ensuing changes in mechanical properties. In this study, extensive molecular dynamics (MD) simulations were performed on the diffusion of the SIA and its clusters in bcc Fe. Diffusivities were calculated for various SIA cluster sizes. It was found that, although the diffusivity itself decreases as the SIA cluster size increases, their activation energy for migration is very small and does not increase with size, in contrast with previous assumptions. Based on previous results obtained by Wirth et al. and the current calculations, we study the mechanism of single SIA diffusion by a kinetic Monte Carlo technique. The resulting model is consistent with experiments. An important conclusion of this study is that the ‘effective’ migration energy of the single SIA (0.17eV in the present MD study) is smaller than the activation energy for stage IE recovery. The proposed model explains all the details of the low temperature recovery stages. ID and IE, of bcc Fe without the need to invoke the existence of two independent interstitial configurations.